1. Field of the Invention
The present invention relates to package structures and methods for fabricating the same, and more particularly, to a quad flat non-leaded (QFN) package structure with an electromagnetic interference (EMI) shielding function and a method for fabricating the same.
2. Description of Related Art
Generally, a lead frame can be used as a chip carrier, which has a die pad and a plurality of leads formed around the periphery of the base. A chip can be mounted to the die pad and electrically connected to the leads through bonding wires. Further, an encapsulant can be formed to encapsulate the chip, the die pad, the bonding wires and the inner portions of the leads so as to form a semiconductor package with a lead frame.
In order to reduce package size, QFN package structures that dispense with outer leads protruding from the encapsulant are developed. However, such a package size can easily be influenced by external EMI noises during operation, thus adversely affecting electrical performance of the overall package structure.
Accordingly, U.S. Pat. No. 5,166,772 discloses a structure with a metal shield embedded in the encapsulant thereof.
FIG. 1 is a cutaway perspective view of the structure disclosed in U.S. Pat. No. 5,166,772. Referring to FIG. 1, a chip 11 is mounted on a substrate 10 and electrically connected to the substrate 10 through a plurality of bonding wires 12, wherein the substrate 10 has at least a ground terminal 14, and a perforated metal shield 13 is disposed to cover the chip 11 and electrically connected to the ground terminal 14. An encapsulant 15 is formed to cover the metal shield 13, the chip 11, the bonding wires 12 and a portion of the substrate 10, thereby embedding the metal shield 13 in the encapsulant 15. The metal shield 13 shields the chip 11 from external EMI so as to improve electrical performance of the overall structure. Similar structures are also disclosed in U.S. Pat. No. 4,218,578, No. 4,838,475, No. 4,953,002 and No. 5,030,935.
However, since an additionally fabricated metal shield 13 is required in the above-described structure, the fabrication process of the structure is complicated. Further, the metal shield 13 is required to cover the chip 11 and fixed to the substrate 10, thus increasing the assembly difficulty. Furthermore, after the metal shield 13 is disposed on the substrate 10 to cover the chip 11, the encapsulant 15 must pass through the metal shield 13 in order to encapsulate the chip 11. Since the metal shield 13 is perforated, when the encapsulant 15 passes through the metal shield 13, turbulence can easily occur in the encapsulant 15, thus resulting in generation of air bubbles in the encapsulant 15 and causing a popcorn effect in a subsequent thermal processing.
FIG. 2 is a cutaway perspective view of a structure disclosed by U.S. Pat. No. 5,557,142. Referring to FIG. 2, a chip 21 is mounted on a substrate 20 and electrically connected to the substrate 20 through a plurality of bonding wires 22. Further, an encapsulant 23 is formed to encapsulate the chip 21, the bonding wires 22 and a portion of the substrate 20, and a metal layer 24 is formed on the exposed surface of the encapsulant 23 through coating or sputtering so as to shield the package structure from EMI. Similar structures are also disclosed in U.S. Pat. No. 5,220,489, No. 5,311,059 and No. 7,342,303.
The above structures dispense with complicated processes. However, since the metal layer 24 must be formed after a singulation process and it is difficult to perform component arrangement and pickup in a singulated package structure, the above structures are not suitable for mass production. In addition, the sputtering process cannot be applied in a package structure in which the encapsulant is flush with the substrate.
In a package structure disclosed by U.S. Pat. No. 7,030,469, a groove is formed on an encapsulant to expose bonding wires, and a conductive wire layer is formed in the groove and on the encapsulant to electrically connect to the bonding wires, thereby achieving a shielding effect. However, the conductive wire layer is made of a non-ferrous metal material and can only be formed on the groove and encapsulant by depositing or sputtering. Therefore, it cannot be applied in a package structure in which the encapsulant is flush with the substrate. Further, the contact between the conductive wire layer and the bonding wires is point contact, which can easily result in poor electrical connection between the conductive wire layer and the bonding wires.
Therefore, it is imperative to overcome the above drawbacks of the prior art.